In the side airbag device described in following Patent Document 1, a side airbag is divided into a first inflating portion (a rear bag portion) and a second inflating portion (a front bag portion) by a partitioning wall (a front-rear partitioning portion) that is provided at the position of the ribs of a vehicle occupant who is at an improper position. An inflator is provided within the first inflating portion, and gas generated from this inflator is supplied to the interior of the front bag portion through a communication hole that is formed at the front-rear partitioning portion. Due thereto, by making the rear bag portion have higher pressure than the front bag portion, the ability at the initial stage to restrain the vehicle occupant who is at the proper position is ensured, and, even if the vehicle occupant is positioned at an improper position (out of position), effects on the vehicle occupant are suppressed.